Massive black holes or stars first: the key is the residual cosmic electron fraction

TL;DR

This paper explores how variations in the residual cosmic electron fraction after recombination influence early structure formation, potentially leading to the formation of massive black hole seeds in the early universe.

Contribution

It demonstrates that lower residual electron fractions delay baryonic collapse, increasing halo mass and enabling the formation of massive black hole seeds, offering a new pathway for early SMBH formation.

Findings

Lower electron fractions delay collapse and increase halo mass.

Enhanced gas inflow rates facilitate massive seed black hole formation.

A new pathway for SMBH seeds arising from residual electron fraction effects.

Abstract

Recent James Webb Space Telescope observations have unveiled that the first supermassive black holes (SMBHs) were in place at z $\geq$ 10, a few hundred Myrs after the Big Bang. These discoveries are providing strong constraints on the seeding of BHs and the nature of the first objects in the Universe. Here, we study the impact of the freeze-out electron fractions ($f_e$) at the end of the epoch of cosmic recombination on the formation of the first structures in the Universe. At $f_e$ below the current fiducial cosmic values of $\rm \sim 10^{-4}$, the baryonic collapse is delayed due to the lack of molecular hydrogen cooling until the host halo masses are increased by one to two orders of magnitude compared to the standard case and reach the atomic cooling limit. This results in an enhanced enclosed gas mass by more than an order of magnitude and higher inflow rates of up to $0.1~M_{\odot}/{yr}$. Such conditions are conducive to the formation of massive seed BHs with $\sim 10^{4}$ M$_{\odot}$. Our results reveal a new pathway for the formation of massive BH seeds which may naturally arise from free